Acc. Chem. Res. 1994,27, 279-285
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Designing Ligands for Oxidizing Complexes TERRENCE J. COLLINS Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213 Received April 6, 1994
Higher oxidation state complexes of transition metals become progressively rarer the further to the right the element is found in the periodic table. For early transition metals the do states are the principal ones found in nature. For the middle transition metals (first row: Cr, Mn, Fe), the do states are found in small numbers of mostly synthetic, reactive oxidants. For later transition metals (first row: Co, Ni, Cu), the do states are unknown in stable compounds. Nearly all middle and later transition metal compounds have comparatively low formal oxidation states. For example, the Chemical Abstracts Formula Index lists more than 102 000 copper compounds, alloys noninclusive, that were identified from 1965 t o 1990. Copper possesses 11valence electrons, but +IV is the highest state observed in a coordination complex, and it is found only in [cd?6]2-.1 For iron, more than 115 000 compounds were characterized in the same and period, but do Fe(VII1) is known only in Fe04,2~3 one can estimate that fewer than 0.2% of iron compounds have oxidation states greater than +III. Nevertheless, high oxidation state middle transition metal compounds are important in chemistry and biology as oxidants. The first goal of our program was to learn how to make high oxidation state and strongly oxidizing middle and later transition metal compounds that are stable. This has provided direction for our research in ligand design, a direction that incorporates our second goal: learning how t o make better homogeneous oxidants. I will begin by pointing out an important distinction between the two roles transition metal ions play in oxidation reactions. In metal-based oxidizing agents, the metal can have one of two principal roles.4 (i) When a metalloredoxactive oxidant oxidizes a substrate, the event is accompanied by a formal oxidation state reduction at the metal center. We originally defined the term p r i m a facie metallo-oxidant for this class, but now ~ When a metprefer metalloredox-active ~ x i d a n t .(ii) allotemplate oxidant oxidizes a substrate, the metal does not undergo a formal oxidation state change. Instead, the metal activates the primary oxidant and/ or substrate and/or arranges the primary oxidant and substrate in a favorable geometry for oxidation to proceed. Permanganate5~6and chromium(VI) compounds5,' are classic examples of metalloredox-active oxidants in chemistry. As examples in biology, metalloredox-active oxidants are found at the active site of cytochrome P-450enzymes, where the weight of evidence indicates that iron(IV) oxoporphyrin radical Terrence J. Collins is Professor of Chemistry at Carnegie Mellon University. He MSc. (1975),and Ph.D. (1978)degrees from the University earned his BSc. (1974), 01 Auckland in New Zealand, where he worked with Warren R. Roper. After postdoctoral work at Stanford University with James P. Collman, he joined the faculty of Caltech in 1980 and the faculty of Camqie Mellon University in 1988. His research interests are focused on high oxidation state middle and later transition metal chemistry with emphasis on ligand design for biomimetic chemistry, the development of robust, recyclableoxidants, and the constructionof multinuclear ions with predictable magnetic properties.
cations oxidize many different substrates,* and in methane monooxygenase, where a high-valent oxobridged diiron species oxidizes methane to methan~l.~-l~ The enantioselective oxidations of prochiral allylic alcohols by TBHP catalyzed by titanium tartrate complexes provide examples of metallo-template oxid a t i o n ~ .Recognizing ~~ these two classes of metallooxidants is important because the further development of each class appears to present different challenges. Major achievements such as the Sharpless systems13 have been made in metallotemplate oxidations with comparatively mild processes where weak C-H bonds and other easily oxidized bonds in ligands are clearly tolerable. The sensitivity of ligands to oxidative decay has not confined metallotemplate oxidation chemistry perhaps because the metal center is not directly part of the oxidizing entity. The real oxidant is further removed from ancillary ligands than in metalloredoxactive oxidants. Some of the frontier challenges in metalloredoxactive oxidant chemistry are demanding oxidations such as the selective oxidation of unactivated C-H bonds. These problems require that ligand systems be especially inert to oxidation. It is difficult for chemists to follow nature's approach to solving the ligand sensitivity problem for finding effective metallooxidants for demanding oxidations. Nature efficiently catalyzes intrinsically slow substrate oxidations by using numerous selectivity-directingfeatures that are extrinsic t o the reaction itself. Reaction sites and transition states are crafted not only to accelerate desired reactions but also to exclude undesired processes such as attack at the ligand system and the protein. Our work is focused on developing ligands that might lead to improved metalloredox-active oxidants for demanding oxidations. In the early 1980s, ligand design looked to be well worth exploring as a tool for expanding high oxidation state middle and later transition metal chemistry and (1)Kissel, D.; Hoppe, R. Z . Amrg. Allg. Chem. 1988,559,4048. (2)Kiselev, Y. M.; Kopelev, N. S. Dokl. Chem. 1987,292,29-31. (3)Kiselev, Y. M.; Kopelev, N. S.;Bobylev, A. P. Russ. J . Inorg. Chem. 1989,34,1517-1520. (4)Collins, T.J.; Gordon-Wylie, S.W. J . Am. Chem. SOC.1989,111, 4511-4513. (5)Benson, D. Mechanisms of Oxidation by Metal Ions; Elsevier Scientific Publishing Co.: Amsterdam, 1976. (6)Arndt, D. Manganese Compounds as Oxidizing Agents in Organic Chemistry; Open Court: La Salle, IL, 1981. (7)Cainelli; G.; Cardillo, G. Chromium Oxidations in Organic Chemistry; Springer-Verlag: Berlin, 1984. (8)Ortiz de Montellano, P. R. In; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York, 1986;pp 217-271. (9)Fox, B. G.;Froland, W. A.; Dege, J. E.; Lipscomb, J. D. J . Biol. Chem. 1989,264,10023-10033. (10)Fox, B. G.; Borneman, J. G.; Wackett, L. P.; Lipscomb, J. D. Biochemistry 1990,29,6419-6427. (11)Que, L. J. In Bioinorganic Catalysis; Reedijk, J., Ed.; Marcel Dekker: New York, 1993;pp 347-393. (12)Feig, A. L.; Lippard, S. J. Chem. Rev. 1994,94,759-805. (13)Finn, M. G.;Sharpless, K. B. J . A m . Chem. Soc. 1991,113,113126 and references therein.
0001-484219410127-0279$04.50/0 0 1994 American Chemical Society
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280 Acc. Chem. Res., Vol. 27, No. 9, 1994
as a step toward better metalloredox-active oxidants. In seminal work, Dale Margerum and his group had shown that the deprotonated amide ligands of small polypeptides could be used to stabilize Cu(II1)and Ni(111)thermodynami~a1ly.l~ Furthermore, Margerum showed that the secret of this conveyed stability was the strong amido-N a-donor capacity which correlates well with the pKa of the parent amide.15 The polypeptides decomposed in the oxidized complexes, but the decomposition could be blocked when the ligand C-H groups were replaced with C-CH3.16 Chang and Ebina introduced polyfluorination of porphyrins t o give oxidation catalysts that were much longer lived than the hydrogen-substituted analogues,17 thereby laying the foundation for wide investigations into protected-porphyrin-based oxidizing systems.18 We had set out t o look for ligands that are resistant to oxidation and also strongly d ~ n a t i n g . ’ ~ - We ~ l expected that incorporating these properties in ligands would allow us to develop high-valent complexes with normal shelf lives. We also expected to be able to manipulate reactivity by changing metal ions and by modifvingligand donor properties. The search has led to a still expanding group of concepts that are useful for guiding ligand development in this area,22and it has provided us with unexpected opportunities to contribute to knowledge.
The Ligand Design Approach Design-oriented coordination chemists generally view a chelating ligand as a collection of adjustable components: the donor atoms and their basicities; the donor functional groups and their spatial orientation and electronic and steric properties; the chelate rings and their sizes, structures, strains, and relationships to each other; the ring substituents and their electronic and steric effects; the acyclic or macrocyclic nature of the chelate; the cavity size of a macrocycle; etc. Ligand design can be viewed as the process by which the components are varied either for curiosity’s sake or to control the properties of a targeted system. The process has a distinguished history in, for example, macrocyclic ligand c h e m i ~ t r y ,crown ~ ~ , ~ether ~ and related chemistry,25and porphyrin chemistry.26 Here, only one part of the design process for our (14)Margerum, D. W. Pure Appl. Chem. 1983,55,23-34. (15)Bossu, F. P.; Chellappa, K. L.; Margerum, D. W. J. Am. Chem. SOC.1977,99,2195-2203. (16)Diaddario, L. L.; Robinson, W. R.; Margerum, D. W. Inorg. Chem. 1983,22,1021-1025. (17)Chang, C. K.;Ebina, F. J. Chem. SOC.,Chem. Commun. 1981, 778-779. (18)Chen, H. L.; Ellis, P. E., Jr.; Wijesekera, T.; Hagan, T. E.; Groh, S. E.; Lyons, J . E.; Ridge, D. P. J. Am. Chem. SOC.1994,116, 10861089 and references therein. (19)Anson, F. C.; Christie, J. A,; Collins, T. J.; Coots, R. J.; Furutani, T. T.; Gipson, S. L.; Keech, J. T.; &a%, T. E.; Santarsiero, B. D.; Spies, G. H. J.Am. Chem. SOC.1984,106,4460-4472. (20)Christie, J. A,; Collins, T. J.; KrafR,T. E.; Santarsiero, B. D.; Spies, G. H. J. Chem. Soc., Chem. Commun. 1984,198-199. (21)Collins, T.J.; Santarsiero, B. D.; Spies, G. H. J.Chem. Soc., Chem. Commun. 1983,681-682. (22)Collins, T.J.; Kostka, K L.; Uffelman, E. S.; Weinberger, T. I m r g . Chem. 1991,22,4204-4210. (23)Coordination Chemistry of Macrocyclic Compounds; Melson, G. A,, Ed.; Plenum Press: New York, 1979. (24)Lindoy, L. F. The Chemistry of Macrocyclic Ligand Complexes; Cambridge University Press: Cambridge, 1989. (25)Cation Binding to Macrocycles. Complexation of Cationic Species by Crown Ethers; Inoue, Y., Gokel, G. W., Eds.; Marcel Dekker, Inc.: New York, 1990. (26)The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978.
ligands is highlighted: the protection of ligands from oxidative degradation. Generally, the frontier orbitals of an oxidizing transition metal complex are primarily metallic in character. Usually, one can design so-called “innocent” n ligand complements to ensure that this is the case. In such a complex, the oxidizing metal ion is easily reduced if the closest source of reducing equivalents, the ligand system, can be oxidized in any way. Therefore, three primary questions about ligand oxidative degradations are evident (1)What reactions are possible for any specific ligand complement? (2) Are individual reactions general to a range of ligand systems and metal ions? (3) Can specific reactions be blocked by ligand modifications? An important secondary question concerns whether a ligand oxidative degradation is intramolecular or intermolecular. Iterative design has been employed to probe these questions. One starts with a polydentate ligand thought to be suitable. A metal is coordinated in a low valent state, and the complex is chemically or electrochemically oxidized by one or more electrons in an inert medium. If the oxidized complex so produced is unstable, the degradation process is studied. Complete mass balance is sought t o assist in understanding the fate of the ligand and the metal. When the sensitive ligand group has been identified, it is replaced with a substitute thought to be less sensitive and the process is repeated. If the high-valent complex produced by oxidation is stable, it is fully characterized, then oxidized again to induce ligand decay. The process stops when oxidatiodigand degradation cannot be observed using chemical or electrochemical techniques in carefully dried, oxidation resistant media. Oxidation processes have been observed at more positive potentials than 2 V (SCE)in liquid SO2 solvent for the systems studied.28
The Ligand Protection Rules The three primary questions posed above can be answered as follows: (1)It has been possible t o trace each found oxidative degradation to a specific ligand group. (2) Individual electron-releasing ligand reactions appear to point t o general principles of ligand protection. (3) As a general rule, specific electronreleasing ligand reactions can be blocked by appropriate ligand modifications. To stimulate the process of unifying the chemistry of oxidative ligand degradations with general concepts, we expand here on previously suggested ligand protection rules.22 The rationalizations of the ligand degradations that lead to rules I, 11, and I11 are mechanistic and cannot be proven to be correct, only t o be wrong. Rule I. For chelate rings, a hydrogen atom should not be placed on an atom that is p to an oxidizing metal center, if the a-atom can support an increase in the bond order with the p-atom. Degradations resulting from this feature are well-established in the literature. There are two types. In the first, the redox process is stoichiometrically localized on the ligand, e.g., amine to imine. Such processes are often viewed as useful syntheses of new ligands, and many are considered (27)Jorgensen, C. K.Structure Bonding 1966,1, 234-249. (28)Anson, F. C.; Collins, T. J.; Gipson, S. L.; Keech, J. T.; &am, T. E. Inorg. Chem. 1987,26,1157-1160.
Acc. Chem. Res., Vol. 27, No. 9, 1994 281
Designing Ligands for Oxidizing Complexes
c
c + denotes a compound ChaTaClefhd by Bn X-tay crystal structure determination
C
4
Starting Material
ROH, -2e-Y
I
- 2 i , 2H20 -2H+,-2ROH
-H+ /+w
hydrolysisalcoholysis
c
12+
hydrolysis-
%I
Figure 1. Characterized intermediates in degradation of a diamido-N-diphenoxido acyclic chelate.
to be radical in nature.29 The examples we discovered (Figure 1) are of a second type, where the redox process occurs at both the metal and the ligand. These ligand degradations appear to be best formulated as two-electron processes, and eq 1shows what happens to the sensitive portion of one of the ligand^.^^^^^
Remote OMRE reactions appear to be widely distributed in transition metal chemistry and include higher order processes (1,5etc.). The example in eq 1points t o the facile nature of the 1,3-RE when the departing two-electron acceptor is a proton. The degradation can be blocked by aromatizing the C-C bondlg or substituting alkyl for H.16 Rule 11. A heteroatom should not be attached to a five-membered chelate ring on an atom that is y to an oxidizing metal center, i f the heteroatom has an available lone pair to stabilize forming cationic character on the y-atom as the endocyclic B-y bond is oxidatively cleaved by the metal. This rule may point to a more facile degradation than does rule I.19,30831The example shown in eq 2 is a remote 1,3-RE where the twoelectron acceptor in the reverse 1,3-OAis a carbocation.
Here it is useful to introduce a concept to emphasize the possible presence of communication channels comprised of overlapping orbitals between the metal ion and remote ligand sites that establish favorable conditions for oxidative ligand degradations. The reaction of eq 1 can be conceptualized as a remote reductive elimination, in this case a 1,3-reductive elimination (1,3-RE), with the /%carbon atom in the 1-position and the metal atom in the 3-position. Removal of the proton results in a formal two-electron reduction of the metal. The microscopic reverse is an example of a remote oxidative addition, in this case a ..+ ! l,&oxidative addition (1,3-OA). The concept can be generalized. Two-electron remote oxidative addition reactions are possible whenever a remote ligand site is able to formally withdraw two electrons from the metal ion by bonding to a two-electron acceptor. The j steps _.. conditions for a remote oxidative addition reaction................................t appear to be most favorable when the remote site is conjugated with the metal. One-electron remote O N Rule 111. A heteroatom should not be employed as an a-donor atom i n a five-membered chelate ring, i f it RE reactions can be defined in a parallel fashion. has an available lone pair to stabilize forming cationic (29) Endicott, J. F.; Durham, B. In Coordination Chemistry o f M a c character on the p-atom as the endocyclic P-y bond is rocyclic Compounds; G. A. Melson, Ed.; Plenum Press: New York,1979; 2
pp 393-460. (30)h s o n , F. C.; Collins, T. J.; Coots, R. J.; Gipson, S. L.; Krafff, T. E.;Santarsiero, B. D.; Spies, G. H. Inorg. Chem. 1987,26, 1161-1168.
-
(31)Anson, F. C.; Collins, T. J.; Gipson, S. L.; Krafft, T. Chem. 1987,26, 731-736.
E.Inorg.
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Collins
co(r4-2)
1
Hd21
[FeCI(r4-2)J
(cO(lC4-2)).
[Ni(r4-2)J
Figure 2. Structurally characterized complexes of macrocyclic tetraamido-N ligands.
oxidatively cleaved by the metal. This rule derives from a process proposed t o account for a slowly degrading oxidation catalyst (eq 3).32 We surmise that the 0, heteroatom stabilizes forming cationic character at the /?-carbon in the transition state of the proposed degradation. The process can be viewed as a 1,3-RE with a coordinated acetone zwitterion functioning as the two-electron acceptor in the reverse 1,3OA. Photochemical decomposition of a planar Co(II1) complex containing the five-membered ring produces acetone.33 The ligand sensitivity appears to be effectively checked when the alkoxide donor is replaced by an amido-N donor. The amido-N lone pair is orthogonal to the C-C 8-orbital in the planar form of the chelate ring and can only be marginally available to stabilize cationic character at the /?-carbon when the ring distorts. The lone pair availability has been restricted as much as possible by placing the fivemembered ring in a m a c r ~ c y c l e . ~ ~
Rule IV. If the goal is to produce a strong electrontransfer oxidant, amido-N donors should be avoided as internal donors in acyclic chelate ligands. When all electron-releasing, bond-breaking processes have been identified and blocked in amido-N-containing chelates, intramolecular isomerization processes can occur to produce nonplanar amido-N ligands to stabilize oxidizing metal centers (eq 4). The isomerizations can also compete with ligand degradation reactions. 19,30*31 Nonplanar amido-N ligands are much stronger donors than their planar counterparts, and this difference provides the driving force for the discovered isomerization^.^^ Our systems provided the first distinctly nonplanar amides in inorganic chemi s t ~ The . ~ studies ~ were extended to give nonplanar amides in three separate classes: (I) the class just described where nonplanar amides are formed reversibly because of electronic demands at the metaP5a6
probably via an intramolecular twist mechani~m?~ (11) amides that are nonplanar because of ring constraints (111)amides like those in organic c h e m i ~ t r y , ' and ~,~~ that are nonplanar as a result of steric effects.39 Although formation of class I nonplanar amides provides a fascinating vignette on ligand adaptability when the metal has need of additional electron density, the isomerizations complicate the study when the purpose is simply to obtain stable, strongly oxidizing complexes. The isomerizations can be blocked by enclosing amido-N donors in macro cycle^.^^
The combination of these four rules brings us through a number of ligand generations t o the macrocyclic tetraamides of Figure 2,*O H4[1Iz2 and H4[2].34 These ligands are leading to the planned contribution to high oxidation state middle and later transition metal chemistry. Their oxidation resistance also permits the isolation of strongly oxidizing complexes.34 The ligand Hd11 with its all-aliphatic ring systems is an "innocent" 27 ligand. A detailed discussion of ligand (32)Collins, T.J.; Ozaki, S.; Richmond. T . G . J . Chem. SOC.. Chem. Commun. 1987,803-804. (33)Lee, G.H.; Larson, J. L.; Perkins, T. A,; Schanze, K. S. Inorg. Chem. 1990,29,2015-2017. (34)Collins, T. J.; Pow.ell, R. D.; Slebodnick, C.; Uffelman, E. S. J . -842.5 Am. Chem. SOC. 1991,113,8419(35)Anson, F.C.; Collins, T. J.;Ggion, S. L.; Keech, J. T.; Kraftt, T. E.; Peake, G. T. J.Am. Chem. Soe. 1986,108,6593-6605. (36)Collins, T.J.;Coots, R. J.; Furutani, T. T.; Keech, J. T.; Kraftt, T. E.; Peake, G. T.; Santarsiero, B. D. J . Am. Chem. SOC.1986, 108, 5333-5339. . . ~~.~ ~ ~ (37)Collins, T.J.;Keech, J. T . 3. Am. Chem. SOC. 1988,110,11621167. (38)Collins, T.J.; Uffelman, E. S. Angew. Chem. 1989,101, 15521554. (39)Collins, T.J.;Lai, T.; Peake, G . T. Inorg. Chem. 1987,26,16741677. (40)For a summary of work performed with macrocyclic ligands containing one, two, or three amides, see: Kimura, E. J . Coord. Chem. 1986,15,1-28.
Designing Ligands for Oxidizing Complexes
[My~~-2)]"-
ACC.Chem. Res., Vol. 27, No. 9, 1994 283
I
[M"**(d-2)]"-
Figure 3. Resonance contributors in a n-delocalized noninnocent ligand complex.
I
L
innocencehoninnocence is outside the scope of this Account, but from a coordination chemist's point of view, H4[11 is considered to be innocent because the nitrogen donors are electronegative and because there is only one reasonable closed-shell form in which the ligand can be conceptually removed from the metal, i.e., Ill4-. Thus, the formal oxidation state in high valent complexes is considered to be unambiguous. By comparison, H4[21 is considered to be a "noninnocent" 27 ligand because an oxidation site can be localized on the metal or on the phenylenediamido-N unit or delocalized over both. The ligand system can be removed in two reasonable closed-shell forms, i.e., [214and [212-(Figure 3). The innocence of [114- and noninnocence of [214-in complexes have been probed by a number of techniques such as Mossbauer spectroscopy and X-ray crystallography (see below). In many ways, H4[2] is more interesting than H4[1] because the metal and the noninnocent component of the ligand can be viewed as an adjustable delocalized site for redox processes. I will now interrupt the ligand design theme to highlight a rather obvious problem with decaying oxidation catalysts that is apparent from the longest ligand oxidationhydrolysis decay sequence uncovered employing the strategy discussed above.
A Paradox with Decaying Oxidation Catalysts The sequence of Figure 1is shown not for detailed analysis, which is not possible within the space constraints of an Account, but instead to alert the reader to a general concern with metal-catalyzed oxidation processes where the metal catalyst degrades. The sequence was found when the osmium(IV) starting material was tested as a potential electrocatalyst for alcohol oxidations. A catalytic system was obtained, but the first flow of current does not emanate from catalytic alcohol oxidation. Instead, it results from a systematic degradation of the ethylene group of the starting complex's five-membered chelate ring. One can assert not only that the characterized species of Figure 1 are present in the degradation sequence but also that their one-electron-oxidized products are present, showing that at least 30 discrete species are involved from the starting material to the final to proobservable c o m p l e ~ e s .An ~ ~isomerization ~~~ duce nonplanar amido-N ligands (see rule IV)occurs in the middle of the sequence leading to two parallel diastereomericbranches for further degradation. Other intermediates were formed in the degradation seauence in concentrations that were too low to allow fbr isolation and complete characterization. The two final observed species, labeled as such in the sequence, are the principal materials present while the alcohol is undergoing electrocatalyticoxidation. However, the system undergoes further degradation to intractable species that are not catalytically active.
Figure 4. czv symmetry of an intermediate spin (S= I), planar, four-coordinate cobalt(II1)complex showing estimated ordering of frontier molecular orbitals.
The detection and characterization of this very complex degrading oxidation catalyst system highlights an obvious paradox. If the catalyst system decays to inactive materials, any single degradation intermediate or any combination of intermediates could be the actual catalyst or catalysts. If the slowest step in a decay sequence comes first, the rate of loss of catalytic activity will coincide with the rate of loss of the starting material, although the ligands of the catalytically active species could be substantially different from those of the starting material. Characterization of another intermediate in a decay sequence preceding the catalytically active complex simply takes one to a new starting material with an uncertain relationship to the actual catalyst. For example, in the electrocatalytic alcohol oxidation just described, we do not know that any residual portion of the starting chelate is coordinated to the true catalyst.
Developing High Oxidation State Middle and Later Transition Metal Chemistry The tetraanions [lI4- and [214-bond to the first-row metals, Cr through Cu. Metal insertion procedures typically give yields of 290% after isolation. These ligands and earlier generation acyclic diamido-Ndialkoxido analogues41present a very strong planar, four-coordinate, a-field to each metal ion. The interaction occurs primarily with just one metal d-orbital, which thus becomes distinctly a-antibonding. This orbital is never found to be occupied for d-electron counts less than nine. The spin state usually observed is obtained by distributing the metal d-electrons according to Hund's rule among the remaining four d-orbitals. The isolation at high energy of the one metal d-orbital that becomes strongly a-antibonding is represented in Figure 4 for a structurally characterized, intermediate spin (5' = l),planar, four-coordinate cobalt(II1) ~ o m p l e x . ~ ~ ~ ~ ~ Derivative high-valent complexes of the tetraamide macrocycles are stable under ambient conditions in appropriate solvents and in the solid state. Each of the representative compounds shown in Figure 2 has been structurally characterized. Most are remarkably resistant to hydrolysis. Anion [Mn(O)t~~-3)]-, with an early generation acyclic ligand, was the first (41) h s o n , F. C.; Collins, T. J.; Richmond, T. G.; Santarsiero, B. D.; Toth, J. E.;Treco, B. G. R.T.T. J.Am. Chem. SOC. 1987,109, 29742979. (42) Gordon-Wylie, S.W.; Bominaar, E. L.; Collins, T. J.; Workman, J. M.; Claus, B. L.; Patterson, R. E.; Williams, S.A.; Conklin, B. J.;Yee,
G. (43) T. Submitted Brewer, J.toC.; J . Collins, Am. T. J.; Smith, M. R.; Santarsiero, B. D. J. Am. Chem. SOC.1988,110,423-428.
204 Acc. Chem. Res., Vol. 27, No. 9, 1994 isolated manganese-monooxo ~ o m p l e x .It~is subject
to hydrolysis on standing in water, but [Mn(0)(tc4-2)1is not, allowing for 0-atom exchange of the oxo ligand with water.44 This illustrates an important design principle. Hydrolysis of [Mn(0)(tc4-3)1-probably proceeds via initial protonation of the alkoxide donors. A similar process is not possible in [Mn(0)(tc4-2)]-as there is no basic site for protonation to initiate the decomplexation process. The anions, [Mn(O)(~~-3)1and [Mn(O)(tc4-2)I-,are not strong 0-atom transfer agents, presumably because of the high donor capacity and negative charge of the chelates. The Cr(V) analogues, [Cr(0)(tc4-l)l-and [Cr(0)(tc4-2)1-,are also not reactive 0-atom transfer agents toward olefins.45 However, these very stable chromium compounds cleave DNA.46 We have found that it is not necessary to change the ligand systems significantly to get oxidizing species in the presence of organic substrates and peroxides as hinted at below. The complex [FeIVC1(tc4-1)]-is the first example of high-spin (S = 2) iron(IV)in a homogeneous coordination complex.47@The dianion [Fen1C1(tc4-1)I2contains iron(II1)in the rare intermediate spin state (S= 3/2).47 The isocyanide complex Fe"(CNtBu)2(tc4-1) is a sixcoordinate, intermediate-spin (S = 1)iron(IV)complex of a potentially strong n-acid where the n-acidity is not expressed.49 The compound's stability indicates that the FeIV(tc4-1)unit is indiscriminate in terms of the axial ligands it will accept. Compounds [FeIVC150 and Few(HzO)(~4-2),51 containing noninno(~~-211cent macrocyclic tetraamides, are better considered as species intermediate between Fe(IV) and Fe(II1). Comparisons of their structural and Mossbauer properties with those of their Fe(II1) partners and of the innocent macrocyclic tetramide complexes, [FeNC1(tc4l)I-, [Fe"'C1(tc4-1)I2-,and FeIV(CNtBu)2(tc4-1), indicate that the potential n-noninnocent character of the phenylenediamido-N units is expressed in the two species to different degrees. The chemistry of planar, four-coordinate cobalt(II1) In some has been considerably developed.32,34,38,43a52,53 cases it has been shown that planar Co(II1)complexes can catalyze 0-atom transfers,32including enantioselective ones,54implicating the intermediacy of a Co(V)-oxo complex that retains a significant portion of the starting chelate (a Co(V)-oxo complex would be isoelectronic with an Fe(IV)-oxo complex). Compounds [C0%~-1)1-,~~ [C0%~-2)1-, and related spe(44)Collins, T.J.; Powell, R. D.; Slebodnick, c . ; Uffelman, E. S. J . Am. Chem. SOC.1990,112,899-901. (45)Collins, T. J.;Slebodnick, C.; Uffelman, E. S. Inorg. Chem. 1990, 29,3433-3436. (46)Dillon, C. T.;Lay, P. A,; Bonin, A. M.; Dixon, N. E.; Collins, T. J.; Kostka, K. L. Carcinogenesis 1993,14,1875-1880. (47)Kostka, K L.;Fox, B. G.; Hendrich, M. P.; Collins, T. J.; Rickard, C. E. F.; Wright, L. J.; Miinck, E. J . Am. Chem. SOC.1993,115, 67466757. (48)Collins, T.J.;Kostka, K. L.; Miinck, E.; Uffelman, E. S. J . Am. Chem. SOC.1990,112,5637-5639. (49)Collins, T.J.; Fox, B. G.; Hu, Z. G.; Kostka, K. L.; Miinck, E.; Rickard, C. E. F.; Wright, L. J. J . Am. Chem. SOC.1992,114, 87248725. (50)Bartos, M. J.;Gordon-Wylie, S. W.; Collins, T. J.; Fox, B. G.; Miinck, E.; Weintraub, S.; Wright, L. J. Manuscript in preparation. (51)Bartos, M. J.; Kidwell, C.; Kaufmann, K.; Gordon-Wylie, S. W.; Collins, T. J.; Clark, G. R.; Munck, E.; Weintraub, S. Angew. Chem.,Int. Ed. Engl., submitted. ( 5 2 ) Collins, T. J.; Workman, J. M. Acta Crystallogr.,Sect. C 1993, C49, 1426-1428. (53)Collins, T.J.; Richmond, T. G.; Santarsiero, B. D.; Treco, B. G. R. T. J . Am. Chem. Soe. 1986,108,2088-2090. (54)Ozaki, S.; Mimura, H.; Yasuhara, N.; Masui, M.; Yamagata, Y.; Tomita, K.; Collins, T. J. J . Chem.SOC.,Perkin Trans. 2 1990,353-360.
Collins
r
n
I+
Figure 5. Recently produced complexes for oxidation studies.
~ i e contain s ~ ~ intermediate-spin (S = 1)centers. The one-electron-oxidized complex, C0(tc4-2),is a hydrocarbon-soluble, solution-stable oxidant with a reduction potential of 1.27 V vs NHE.34 This compound and related species are more accurately formulated as Co(111) complexes with one-electron-oxidizedphenylenediamido-N units than as Co(IV) complexes. The compound [Ni"'(tc4-1)1- is a planar, four-coordinate nickel(II1) species with an innocent ligand complement.55 It is the first structurally characterized example of this geometry for Ni(II1) and has gi > gll in the EPR spectrum. Physical evidence indicates that the analogue [Ni(tc4-2)1-is best formulated as a nickel(111)complex.56
Developments in Oxidation Chemistry As noted in the introduction, one aim of our work is to develop ligands that might lead to improved metalloredox-active oxidants. As we have learned how to make progressively more robust ligands, we have begun exploring their use in oxidation processes. Much of our current work is focused on these studies. Three ongoing developments are presented here. The first is based on the known properties of the oxidized cobalt complex, Co(tc4-2)(Figure 2), and related complexes with different substitution patterns on the aromatic ring.34 We have enhanced the hydrocarbon solubility by increasing the size of the aliphatic substituents as in Co(tc4-4)(Figure 5 ) t o produce strong oxidants that are very soluble in alkanes, and we are exploring uses of these unusual reagents5' The second focus is based on a desire to gain broad control over the redox properties of tetraamido-N macrocyclic complexes without having t o change the basic nature of the oxidation-resistant ligand system. This problem is being attacked by developing switching ligands. Switching ligands are ligands which coordinate t o a primary metal site, say a potential 0-atom transfer site, and have one or more secondary sites, called switching sites, for the reversible acquisition of charge. The idea is to change the reactivity at the primary site by coordination of a variety of ions or by electron transfer events at crafted switching sites. Switching ligands offer the promise of metallooxidants with chameleon-like electronic properties. For example, switching ligand complexes might be capable of reacting with mild oxo-forming reagents to produce a relatively unreactive oxo complex that would become a reactive 0-atom transfer agent after a switching event. In the structurally characterized cobalt(II1) complex [Co(tc4-6)1- there is a bidentate switching site consisting of the pyridine and adjacent amide-0 donors. Whereas the cobalt(I1)derivative of [Co(tc4-5)1- is able t o reduce 0 2 by an outer sphere (55)Collins, T. J.; Nichols, T. R.; Uffelman, E. S. J . Am. Chem. SOC. 1991,113,4708-4709. (56)Uffelman, E. S.;Collins, T. J. Unpublished results. (57)Patterson, R. E.;Collins, T. J.; Gordon-Wylie, S. W. Unpublished results.
Designing Ligands for Oxidizing Complexes
ACC.Chem. Res., Vol. 27,No. 9, 1994 285
ceptual level. This should stimulate new research process, coordination of the dication [Ru(bipy)d2+ since, even if the concepts suggested here prove to be produces [Ru(bipy)z[Co(~~-4)]]+, which clearly binds 0 2 . Electrochemical studies reveal that the bound 02 less than optimal, they at least provide a starting point for finding the correct concepts to guide researchers molecule can undergo a multielectron reduction under appropriate condition^.^^ in designing superior oxidation resistant ligands. Much more can and should be done to perfect ligands The third focus is on catalytic oxidation processes. for oxidizing homogeneous complexes. Protecting One example is the C-H bond activation chemistry discovered when we attempted to produce [ F ~ ( O ) ( K ~ - ligand systems from oxidative degradation is only one of the apparent themes. Better systems with multiand 2)1-, the iron(V)-oxo analogue of [Crv(0)(~4-2)1functional reaction sites are needed to move closer in [MnV(O)k4-2)1-.This system is the subject of much work and of a manuscript currently in preparati~n.~" biomimetic chemistry to what nature actually does to effect demanding oxidations. So often in chemistry, a challenging redox process requires a diversity of Concluding Remarks electronic properties of a metal ion catalyst that In accord with the purpose of Accounts articles, I cannot be achieved with a static set of ligands. For have focused here on the systems developed in our example, the reduced form of a redox reagent might laboratories. Original systems for stabilizing high need to be electron-rich to react with a primary oxidation state and strongly oxidizing middle and later oxidant such as oxygen t o produce an oxo complex, but transition metal complexes have been developed in then the oxo complex might need to be electron-poor other laboratories. I would like to call the readers' to attack a resistant substrate. In general, nature attention to the recent work in high-valent iron does not choose to attain diverse electronic properties chemistry of Que et al. in producing dioxo-bridged at an enzyme active site by developing a static nonheme species" and Vogel et al.59in metallocorrole ancillary ligand system. Rather, nature handles these chemistry for examples of exciting recent developchallenges by using systems in which sequential ments. reactions are arranged in time and space to achieve, I believe that high oxidation state middle and later among other things, the necessary electronic divertransition metal chemistry remains largely undevelsity.8-12 .& ligand oxidative sensitivity is becoming oped. I hope that young inorganic chemists starting better understood, it is becoming more appropriate to on new careers with a taste for synthesis, spectrosgenerate synthetically ambitious multifunctional oxicopy, and reaction chemistry and prepared t o deal dants to execute targeted reactions by using a number with the implications of working with paramagnetic of reactions t o selectively amount the desired activasystems will adopt this field as their own. Clearly one tion barriers. impediment has been the instability of most ligands when coordinated in oxidizing complexes. It is enI am greatly indebted to the students, postdoctoral fellows, couraging that it appears to be possible to categorize sabbatical visitors, and collaborating scientists whose names individual oxidative ligand degradations at the conappear in the references for their enthusiasm, ideas, and (58) Gordon-Wylie, s. W.; Horwitz, C. P.; Leychkis, Y. P.; Woomer, C. G. Manuscript in preparation. (59) Vogel, E.;Will, S.; Schulze Tilling, A.; Meumann, L.; Lex, J.; Bill, E.; Trautwein, A. X.; Weighardt, K. Angew. Chem.,Int. Ed. Engl. 1994, 33, 731-735.
devoted efforts and especially to Drs. Erich Uffelman, Scott Gordon-Wylie, and Eckard Munck. I also thank the NSF, the NIH, the Rohm & Haas and Dow Chemical Companies, and the Atlantic Richfield Foundation for financial support.
